1. Field of the Invention
The invention relates to a double-sided metal clad laminate, and more particularly to an adhesiveless double-sided metal clad laminate.
2. Description of the Related Art
With the recent rapid progress of miniaturization and high integration of electronic devices using flexible printed circuit boards, there is an increasing demand for double-sided laminates to cope with the trend for lighter and higher-density circuits. Conventional double-sided clad laminates include a PI base film coated with adhesives such as epoxy or urethane resin on both sides. The use of adhesives, however, increases the thickness of the final product, making it undesirable for use in fine pitch circuits. Moreover, adhesives can cause curling or result in poor dimensional stability and solder resistance. To overcome these problems, adhesiveless double-sided clad laminates have been proposed.
There are various methods for fabricating adhesiveless double-sided clad laminate, such as laminating a copper foil coated with polyimide coating on both sides of the foil.
U.S. Pat. No. 5,112,694 discloses a method for making adhesiveless double-sided clad laminate with single-layer polyimide coating by the following steps. A high glass transition temperature (Tg) polyimide resin is prepared and directly coated on a copper foil. After being subjected to thermal imidization, another copper foil is laminated thereon. In spite of the simple process, such a thermoplastic polyimide usually has a very high Tg (>300° C.), and therefore necessitates a lamination temperature of above 380° C., which is higher than the operational temperature of commercial laminating machines, resulting in minimal commercial applications.
WO2004/085146A1 discloses a polyimide film with a two-layer structure including a polyimide resin with low coefficient of thermal expansion and a thermoplastic polyimide resin The polyimide resin with low coefficient of thermal expansion and the thermoplastic polyimide resin are coated on a copper foil to provide a copper foil/low coefficient of thermal expansion polyimide resin/thermoplastic polyimide resin structure. After cyclized by heating, another copper foil is heat pressed on the copper foil/low coefficient of thermal expansion polyimide film/thermoplastic polyimide film structure. It has been found, however, that the double-sided metal clad laminate has an asymmetric structure. Since the coefficient of thermal expansion of the thermoplastic polyimide film is quite larger than that of the low coefficient of thermal expansion polyimide film, the double-sided metal clad laminate is apt to warp during the circuit etching process.
An already proposed double-sided clad laminate includes a polyimide film with a three-layer structure. First, a first thermoplastic polyimide resin is coated on a copper foil. After drying, a polyimide resin with low coefficient of thermal expansion is coated on the first thermoplastic polyimide resin. After drying, Next, a second thermoplastic polyimide resin is coated on the polyimide film with low coefficient of thermal expansion after drying, thereby providing a copper foil/thermoplastic polyimide resin/low coefficient of thermal expansion polyimide resin/thermoplastic polyimide resin structure. After cyclized by heating, another copper foil is hot pressed on the copper foil/thermoplastic polyimide film/low coefficient of thermal expansion polyimide film/thermoplastic polyimide film structure. The obtained double-sided metal clad laminate has a symmetric structure and achieves the objective of flatness. When considering an interface between the two different polyimide films, the solvent residual of each layer should be controlled. As disclosed in U.S. Pat. No. 6,346,298, the solvent residual of the first thermoplastic polyimide resin has to be controlled to between 20-30%, and the solvent residual of the low coefficient of thermal expansion polyimide resin has to be controlled to between 30-50%, and the solvent residual of the second thermoplastic polyimide resin has to be controlled to between 30-50%, in order to overcome the interface problems. Therefore, the process is rather complicated.
Accordingly, there is a need for an adhesiveless double-sided clad laminate which can provide good reliability without needing a complicated process and high lamination temperature.